The present invention refers to a screening device and a method of manufacture thereof as recited in the preamble of appended independent claims. The present invention thereby refers to screening devices, such as screen cylinders or bent or flat screening elements, for screening, filtrating, fractioning or sorting pulp suspensions in pulp and paper making industry or other similar suspensions. The present invention more particularly refers to screening devices of the type comprising a plurality of screen wires positioned at a small spacing parallel to each other, the plurality of screen wires forming a screening surface facing the pulp suspension to be screened and adjacent wires forming screening openings therebetween allowing an accept portion of the pulp suspension to flow therethrough. The screen wires are supported, preferably on the downstream side of the wires, by a support element, such as a support ring or a support bar or a support plate. The screening devices may have various forms, e.g., be flat, bent, cylindrical or conical just to name a few preferred forms.
In known screening devices of this type, the support elements, which form supports for the screen wires, are formed of either solid bars, mainly rectangular or round in cross section, or sheet metal strips bent to V-form or round form, and most typically positioned perpendicular to the screen wires. There is a number of patents discussing this type of screening devices, like for instance, U.S. Pat. No. 3,716,144, U.S. Pat. No. 3,805,955, and U.S. Pat. No. 6,426,003.
The screen wires are generally fastened to the supporting elements by a welding process which gives rise to a number of disadvantages such as variability distortion, thermal stresses and burrs. Especially, when the welding is done on surfaces facing the fiber suspension flow. The heat induced by the welding often cause distortion of the wires and changes in the screening opening width between adjacent wires. It is therefore difficult to get completely uniform screening openings, which means that the efficiency of the screen suffers. Today, when the desired width of screening openings may be as small as 0.1 mm, only minimal distortions are acceptable. The thermal stresses and the burrs may also lead to failure in operation due to the loading on the screening device in the user's process. Such loading may be either in the form of a constant load or a cyclic loading giving rise to failure by fatigue. Burrs may also catch fibers in the suspension, leading to gradual clogging of the screen or filter, or the formation of so called “strings,” which are very detrimental in the user's process.
It has also been suggested, e.g., in U.S. Pat. No. 5,090,721 and U.S. Pat. No. 5,090,360, to connect screen wires of a certain “key”-type cross section into recesses, in the support bar, having a similar “keyhole”-type form. By means of bending the supporting bars into rings, the screen wires are clamped into position. This design, thereby, requires the manufacturing of a number of relatively complicated and therefore expensive recesses. Further, it can only be adapted to circular or round screens. In another known screening device, the screen wires are fastened by looping them around support bars. Such a screen construction is strong, but the looping areas around the support bars are locally closing the openings and thereby reducing throughput of the screen. Also the looped areas tend to have cavities and uneven spots which are facing the suspension potentially causing fiber hang-up. The above difficulties tend to result in poor quality of screening or mechanical weaknesses or to high manufacturing costs.
A further problem concerning ordinary wire screen cylinders, the structure of which is such that the screen wires are supported by circumferential rings, whereby the screen wires carry all the axial loading, is the bending of the screen wires under load. The axial loading is due to the fastening of the screen cylinder into the screening apparatus. Normally, the lower end of the screen cylinder is arranged to lye tightly on a flange at the bottom of the screening apparatus, and the screen is bolted via its upper end ring to an upper flange in the screening apparatus. To ensure tight and secure fastening the screen cylinder-is subjected to a certain amount of axial loading. When the screen wires carry this loading, they may start bending as also the radial loading created by the rotor rotating inside the screen cylinder aids in bending the wires.